1. Field of the Invention
This invention relates to machines for the production of molded parts from foamed plastic, including soft foam plastics such as expanded olefin polymers and including plastic foam particles greatly expanded in frothing, which machines are provided with a molding die to be filled with expanded or with more or less frothed plastic particles under previously determined gas pressure conditions in its cavity, and which machines have a compressed gas operated filling device for the expanded particles which are disposed in the material supply line to the molding die.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.l.97-1.99
While the production of molded parts from rigid foamed plastic is generally well manageable regarding the filling of the frothed plastic particles into the mold cavity, due to the plastic particles still retaining a considerable residual foaming capacity, the production of molded parts from soft foamed plastic and the production of molded parts from rigid foamed plastics of very low density pose the problem of filling the mold cavities under overpressure so that those plastic particles having no or only very little foaming capacity left, make contact with the proper surfaces under sufficient pressure whereby they can be welded together sufficiently by introducing steam.
In this connection, it is known from German Publication DE-AS No. 16 29 316 to fill those plastic particles having no foaming capacity left, e. g. foamed ethylene polymer particles, into a mold cavity and then reduce the volume of the mold cavity to 90 percent-40 percent of the original bulk volume of the foam articles filled in. But this can be done only with very simple molded parts such as cubes or plates and requires relatively complicated molding dies.
A method for the production of molded parts of foamed olefin polymers is known from German Publication DE-AS No. 25 42 452 in which the foam particles are to be introduced into the mold cavity by compressing them to 20 percent-80 percent of the original volume against a back pressure which is to be relieved after the mold cavity is filled and before the plastic particles are heated and fused together. In practice, filling the cavity with the foamed particles against the back pressure presents difficulties not recognized in the German Publication DE-AS No. 25 42 452. In particular, filling a mold cavity adequately in its corner areas and edge areas has been impossible when filling against back pressure in the method known from Publication DE-AS No. 25 42 452.
To overcome the difficulties occurring when filling the mold cavity against back pressure, a machine is known from German Publication DE-OS No. 34 08 434 in which the foam particles to be filled into the mold cavity are measured in relatively small portions in dosing chambers and are "shot" portion by portion into the mold cavity under high pressure. Depending on the size of the molded part to be produced, such portions must be shot consecutively into the mold cavity in large numbers, e.g. 100 such portions. Apart from the high mechanical costs, this consecutive "shooting" of portions into the mold cavity requires long filling times. Since measuring the portions in the dosing chamber is associated with considerable fluctuations and inaccuracies, there result substantial variations in the degree to which the mold cavity is filled, which leads to areas of poor fusing on the molded part if the amount filled in is only slightly too little and to the reflux of foam particles in the supply line if the amount filled in is only slightly too great.
From the former case there results a very high percentage of scrap which is found in practice. The latter case, namely foam particles refluxing in the supply line, causes severe operating breakdowns leading to considerable machine downtime and necessitating costly measures for their elimination. Filling the mold cavity by "shooting in" portions of foam particles out of dosing chambers is not feasible with the necessary reliability if there are mold cavities with several filling points that must be charged because it is then impossible to assign definitely limited areas in the interior of the mold cavity to the individual filling points, and in consequence when the previously fixed number of portions is fed in, a reflux of foam particles will occur at one filling point and insufficient filling at the other filling point will be had with the predetermined number of fedin portions.